The present invention generally relates to medical fluid systems. More specifically, the present invention relates to systems and methods of performing cassette-based dialysis and devices related thereto.
Due to various causes, a person's renal system can fail. Renal failure produces several physiological derangements. The balance of water, minerals and the excretion of daily metabolic load is no longer possible and toxic end products of nitrogen metabolism (urea, creatinine, uric acid, and others) can accumulate in blood and tissues.
Kidney failure and reduced kidney function have been treated with dialysis. Dialysis removes waste, toxins and excess water from the body that would otherwise have been removed by normal functioning kidneys. Dialysis treatment for replacement of kidney functions is critical to many people because the treatment is life saving.
Hemodialysis and peritoneal dialysis are two types of dialysis therapies used commonly to treat loss of kidney function. Hemodialysis treatment utilizes the patient's blood to remove waste, toxins and excess water from the patient. The patient is connected to a hemodialysis machine and the patient's blood is pumped through the machine. Catheters are inserted into the patient's veins and arteries so that blood can flow to and from the hemodialysis machine. The blood passes through a dialyzer of the machine, which removes waste, toxins and excess water from the blood. The cleaned blood is returned to the patient. A large amount of dialysate, for example about 120 liters, is consumed to dialyze the blood during a single hemodialysis therapy. Hemodialysis treatment lasts several hours and is generally performed in a treatment center about three or four times per week.
Peritoneal dialysis uses a dialysis solution or “dialysate”, which is infused into a patient's peritoneal cavity via a catheter. The dialysate contacts the peritoneal membrane of the peritoneal cavity. Waste, toxins and excess water pass from the patient's bloodstream, through the peritoneal membrane and into the dialysate due to diffusion and osmosis, i.e., an osmotic gradient occurs across the membrane. The spent dialysate is drained from the patient, removing waste, toxins and excess water from the patient. This cycle is repeated.
There are various types of peritoneal dialysis therapies, including continuous ambulatory peritoneal dialysis (“CAPD”), automated peritoneal dialysis (“APD”), including tidal flow APD and continuous flow peritoneal dialysis (“CFPD”). CAPD is a manual dialysis treatment. The patient connects manually an implanted catheter to a drain, allowing spent dialysate fluid to drain from the peritoneal cavity. The patient then connects the catheter to a bag of fresh dialysate, infusing fresh dialysate through the catheter and into the patient. The patient disconnects the catheter from the fresh dialysate bag and allows the dialysate to dwell within the peritoneal cavity, wherein the transfer of waste, toxins and excess water takes place. After a dwell period, the patient repeats the manual dialysis procedure, for example, four times per day, each treatment lasting about an hour. Manual peritoneal dialysis requires a significant amount of time and effort from the patient, leaving ample room for improvement.
Automated peritoneal dialysis (“APD”) is similar to CAPD in that the dialysis treatment includes drain, fill, and dwell cycles. APD machines, however, perform the cycles automatically, typically while the patient sleeps. APD machines free patients from having to manually perform the treatment cycles and from having to transport supplies during the day. APD machines connect fluidly to an implanted catheter, to a source or bag of fresh dialysate and to a fluid drain. APD machines pump fresh dialysate from the dialysate source, through the catheter, into the patient's peritoneal cavity and allow the dialysate to dwell within the cavity and the transfer of waste, toxins and excess water to take place. APD machines pump spent dialysate from the peritoneal cavity, though the catheter, to the drain. As with the manual process, several drain, fill and dwell cycles occur during APD. A “last fill” occurs at the end of CAPD and APD, which remains in the peritoneal cavity of the patient until the next treatment.
Both CAPD and APD are batch type systems that send spent dialysis fluid to a drain. Tidal flow systems are modified batch systems. With tidal flow, instead of removing all the fluid from the patient over a longer period of time, a portion of the fluid is removed and replaced after smaller increments of time.
Continuous flow or CFPD systems clean or regenerate spent dialysate instead of discarding it. The systems flow fluid into or out of the patient, through a loop. Dialysate flows into the peritoneal cavity through one catheter lumen and out another catheter lumen. The fluid exiting the patient passes through a reconstitution device that removes waste from the dialysate, e.g., via a urea removal column that employs urease to enzymatically convert urea into ammonia. The ammonia is then removed from the dialysate by adsorption prior to reintroduction of the dialysate into the peritoneal cavity. Additional sensors are employed to monitor the removal of ammonia. CFPD systems are more complicated typically than batch systems.
Hemodialysis, APD (including tidal flow) and CFPD systems can employ a pumping cassette. The pumping cassette typically includes a flexible membrane that is moved mechanically to push and pull dialysis fluid out of and into, respectively, the cassette. Certain known systems include flexible sheeting on one side of the cassette, while others include sheeting on both sides of the cassette. Positive and/or negative pressure can be used to operate the pumping cassettes.
One problem with the pumping cassettes is leakage. If the flexible membranes experience a pinhole or tear, fluid and air can move from one side of the membrane to the other. Movement of fluid from inside the cassette to the inner workings of the machine can damage the machine. Movement of air from the machine into the cassette can compromise the sterility of the fluid pathways defined by the cassette. There are detection systems that determine when fluid leaks from the cassette to the machine. It is more difficult, however, to detect fluid leaking into the cassette.
Another problem with cassette-based pumping occurs when the cassette is loaded improperly into the machine. Proper alignment is important because portions of the flexible membrane must match corresponding machine portions, e.g., pump and valve actuators. Improper loading can lead to undue mechanical stress being placed on the cassette, harming potentially the cassette and/or the actuator. Improper cassette loading will also likely degrade or prohibit performance of the system.
A further dilemma, especially in CFPD, is the coordination of multiple fluid delivery. Cassette-based peritoneal pumping systems that administer fluids continuously to patients are required to withdraw fluid (ultrafiltrate) from and add fluid (concentrate) to a continuously flowing dialysis fluid loop. The additional fluids have typically necessitated additional dedicated pumps, which make the cassette and dialysis machine larger and noisier. Scheduling the operation of multiple pumps also presents a challenge to system implementers.
Another problem associated with cassette-based pumping is the entrapment of air or other gas into the fluid pathways. Air can enter the system via leaking connections, improper priming, faulty tubing and faulty cassettes. Patient therapy also produces various gases that enter the system. Cassette-based pumps are designed to pump fluid, not gas. Moreover, the removal and delivery of fluid from and to the patient needs to be monitored and controlled. Air and gases upset volume measurement systems that assume no air or gas exists in the fluid pathways. Air and gases can also be uncomfortable for the patient and impede proper waste removal.
It is desirable to remove air and gas from the dialysis fluid before the fluid enters the patient. To this end, cassette-based systems have been provided with air or gas vents. A need continues however to provide for more economical venting systems. Further, prior to infusion, the dialysis fluid solution is heated to body temperature, releasing gas from the solution. Known vents do not vent air or gas due to fluid heating. It is also desirable to have a method for detecting air and fluid, so that the volume of both can measured, detecting air for purging and detecting fluid for ensuring proper therapy.